Equalizing preamplifiers are used for amplifying and equalizing small audio signals from such audio signal sources as magnetic tape heads or phonograph cartridges. In equalizing frequency response, the preamplifier will produce a higher gain for low frequencies starting for example at 50 hertz and reducing the gain progressively through the audio frequency range. In addition, there is often a requirement for gain peaking at a particular frequency such as in the range of 10 to 15 kilohertz. There may also be a requirement for steep attenuation of the gain above a certain frequency such as 20 kilohertz because typically in audio systems, frequencies above 20 kilohertz do not contain any useable signal and only contribute to noise in the system.
In prior art equalization preamplifiers with a peaking capability utilizing an operational amplifier, peaking is accomplished with a resonant circuit containing an inductance in the feedback circuit of the operational amplifier. This approach has the disadvantage of using an inductor along with the additional disadvantage of not attentuating the frequencies above the peak frequency due to the impedance of the inductor in the feedback circuit. Another approach would be to use an active low pass filter with adjustable damping, but such a filter would require an additional amplifying stage resulting in a more complex circuit of higher cost.